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In studying the photoelectrochemical properties of TiO 2, Nb 2 O 5, ZnO, SnO 2, In 2 O 3, WO 3, Ta 2 O 5, and ZrO 2 porous semiconductor films sensitized by the ruthenium(II) cis -bis(thiocyanato)bis(2,2‘-bipyridyl-4,4‘-dicarboxylic acid) complex, it was found that the Nb 2 O 5 semiconductor cell had the next highest incident of monochromatic photon-to-current efficiency (IPCE = 18%) compared to the TiO 2 cell and showed the highest open-circuit photovoltage ( V oc ) among them. The V oc of the dye-sensitized cell was proved to be related to the flatband potential of the semiconductor electrode. The Ru dye adhered to the Nb 2 O 5 surface mainly through an ester-like linkage. It is speculated that electrons are transferred mainly through the conjugated orbitals of the ester linkage and semiconductor conduction band and that the TiO 2 and Nb 2 O 5 conduction bands consisting of d-orbitals are more advantageous than those of s-orbitals in attaining the desired IPCE. The IPCE of the Nb 2 O 5 cell was markedly improved by treating the Nb 2 O 5 electrode with Nb alkoxide, and the maximum IPCE was 32% at 548 nm. The overall solar-to-electric energy conversion efficiency was about 2% (AM-1.5, 100 mW/cm 2 ).
Sayama et al. (1998) studied this question.